Quartz clocks keep time by counting the vibrations of an electrically stimulated quartz crystal.

The longer version

Squeezing a quartz crystal makes it generate a voltage, and applying a voltage makes it deform. Feeding that signal back through a circuit drives the crystal to vibrate at a stable frequency, typically tens of thousands of cycles per second. The frequency depends on how the crystal is cut.

Counting those cycles and dividing down gives one pulse per second to drive a display or motor. Temperature and ageing shift the frequency slightly, which is why cheap units drift by seconds a month. The same principle times computers, phones and most electronic devices that need a steady beat. Better units are trimmed against a reference and hold far closer.

Where this entry sits

These facts wander through clocks, calendars, time zones, and the long human effort to measure passing hours. You will find leap years, odd month lengths, ancient sundials, and the reasons modern timekeeping works the way it does. The full list sits on the Time and Calendars page, where every entry is listed in order.